The atomic bomb, the plastic dumping ground we call our oceans, and Psy's Gangnam Style – the human race has a lot to answer for, but according to a study recently published in the journal Science, the extinction of Africa's large mammals during the Plio-Pleistocene period is not one of them.
When our hominin ancestor "Lucy" (Australopithecus afarensis) meandered the African plains (specifically, the area around Hadar, Ethiopia) 3 million years ago, she would have met at least three species of giraffe, two species of rhino, a hippo, and four species of elephant-like animals. Today, the vast majority of these large, plant-eating mammals are resigned to the history books. Scientists are not exactly sure when or why they faced extinction but the blame is usually placed on our hominin relatives.
However, scientists from the University of Utah say this could be wrong. According to the team's research, the timeline simply doesn't fit.
"Our analyses show that there is a steady, long-term decline of megaherbivore diversity beginning around 4.6 million years ago," lead author Tyler Faith, curator of archaeology at the Natural History Museum of Utah and assistant professor in the Department of Anthropology at the University of Utah, said in a statement.
"This extinction process kicks in over a million years before the very earliest evidence for human ancestors making tools or butchering animal carcasses and well before the appearance of any hominin species realistically capable of hunting them, like Homo erectus."
So, if not us, who (or what) is responsible for their demise? Climate change is the number one suspect. Specifically, the researchers believe the extinctions were the result of falling carbon dioxide levels in the atmosphere, which triggered the expanse of grassland at the expense of shrubland.
The team came to this conclusion after analyzing more than 100 fossil assemblages from the last 7 million years, comparing the results to stable carbon isotope records of vegetation structure and herbivore teeth, and independent records on climatic and environmental trends. They found that over the last 7 million years, 28 lineages of megaherbivore species have gone extinct. What's more, additional analysis suggests that this decline in diversity started roughly 4.6 million years ago.
Importantly, this means it began before the emergence of Homo erectus, the human relative that has received the most blame for the extinctions. According to the study, the rate of decline did not increase following their arrival on the scene. Instead, the losses appear to coincide with the expansion of grasslands, likely related to a worldwide fall in atmospheric carbon dioxide in the last 5 million years, noted study co-author John Rowan.
"Low CO2 levels favor tropical grasses over trees, and as a consequence savannas became less woody and more open through time. We know that many of the extinct megaherbivores fed on woody vegetation, so they seem to disappear alongside their food source," he explained.
This news could also put human ancestors out of the line of fire for the loss of African carnivores, which may have faced extinction due to the decline of the megaherbivores (their prey). However, René Bobe and Susana Carvalho from the University of Oxford, also writing in Science, warn we shouldn't leap to too many conclusions.
"The causes of megaherbivore decline are probably complex, multidimensional, and varied across time and space. The precise timing of key hominin behavioral innovations remains poorly constrained by the current archaeological and palaeontological records," they write.